1. Field of the Invention
This invention relates generally to rotating machinery in which articles are to be transported by a vacuum head carrier and positively removed from the vacuum head by a pulse of air pressure with the air pressure pulses and vacuum being delivered to the carrier through a rotary distributor valve. More particularly, this invention relates to rotary distributor valves for delivery of both air pressure and vacuum to selected points.
2. Description of the Prior Art
Rotating machinery, which utilizes a rotary valve for the delivery of both vacuum and air pressure to selected points, for the positioning of articles is shown in U.S. Pat. No. 3,453,714 by Clark et al. This patent is assigned to the same assignee as the present invention. This patent shows a vacuum operated chip placement machine which is provided with a rotating turret having a multiplicity of arms connected to a distributive valve for delivering metered vacuum to selected ones of the arms at selected times so as to pick up the multiplicity of chips. The vacuum being delivered through the valve tends to cause the sliding parts of the valve to tightly adhere to one another, the sliding friction forces between the parts of the valve is greatly increased requiring greater power to accurately position the arms of the turret.
U.S. Pat. No. 3,973,682, also assigned to the same assignee as the present invention, discloses a pick up apparatus mounted on a rotary spider. The pick up apparatus shown in this patent requires the selective application of both pressure and vacuum to the pick up head. In one such a case first air and then vacuum and then both air and vacuum is delivered to the pick up head through one or more rotary valves. When a single valve is used for both air and vacuum distribution the accurate positioning of the turret arms to the same point each time they arrive at a selected area becomes increasingly difficult because of the variation in pressure between the plates and the valve. For example, when vacuum is being applied, the sliding parts of the valve are held tightly together and the frictional loss between the sliding parts of the valve is greatly increased, but when air pressure alone is being applied, the air pressure forces the sliding parts apart reducing the friction loss. When air pressure is applied at one point in the valve and vacuum at another point in the valve, a still different frictional load is realized. When the valve, as shown in the prior art, is part of the turret these changes in friction in the valve results in uneven and uncontrollable variations in the position of the arms. That is, when there are heavy frictional losses the turret may undershoot the desired position and when the losses are low or nonexistent the turret may overshoot the desired position. Thus in the prior art mechanisms the positional accuracy of the product is affected by the frictional loads on the valve.